The invention relates to a technique for mounting a semiconductor device.
There is an increasing demand for semiconductor devices such as BGA, CSP, WPP, flip chip and etc. each of which semiconductor devices has bumps as external pads. In the semiconductor device having the bumps as the external pads, the bumps are formed by, for example, the steps of: preparing a pad pattern in which the plating of nickel-gold (Ni/Au) and nickel-palladium-gold (Ni/Pd/Au) is applied on the surface of copper; coating the pad pattern with flux by printing; mounting solder balls on the flux-coated pad pattern; and reflow-heating them so that the bumps may be formed.
Further, the mounting of these semiconductor devices is performed by the steps of: coating a paste material, which comprises solder particles and flux, on a pad pattern formed on a substrate; positioning the bumps of the semiconductor device and the pad pattern of the substrate; loading the positioned bumps on the pad pattern; and soldering them.
Usually, a material used for the bumps or the mounting is so-called Snxe2x80x94Pb eutectic solder of Sn-38 mass % Pb.
In recent years, it is urged to put Pb-free solder to practical use.
The melting point of the conventional Snxe2x80x94Pb eutectic solder is 183xc2x0 C. On the other hand, in a Pb-free solder material such as, for example, Anxe2x80x94Agxe2x80x94Cu solder, the melting point thereof ranges from 216xc2x0 C. to 227xc2x0 C., which melting point is higher than that of the conventional Snxe2x80x94Pb eutectic solder.
According to the research performed by the inventors of the invention, it is found that, in a case where a semiconductor device such as BGA which device has bumps is mounted on a substrate, there occurs such a phenomenon as the solder bumps do not always melt readily. Namely, according to the inventors"" research, it is found that a temperature between the semiconductor device and the substrate is lower than the temperature of the substrate or the semiconductor device itself by a value of 15 to 20xc2x0 C.
Thus, by setting the reflow temperature so that these Pb-free solder bumps may be melted, the temperature of the substrate or the semiconductor device itself becomes higher by the value of 15 to 20xc2x0 C. than the temperature measured between the semiconductor device and the substrate, that is, it becomes 240 to 260xc2x0 C., so that there arises a problem of heat resisting property regarding the substrate and other electric parts (such as electrolytic capacitors and etc.) mounted on the substrate.
The object of the invention is to realize solder bonding of high reliability in which the heat resisting properties of a circuit substrate and electronic parts are taken into consideration.
In the invention, in order to achieve the object, there are provided the semiconductor module and the circuit substrate both recited in the claims.
The inventors of the invention have noted the respects that, in a case of mounting on a substrate the semiconductor device such as BGA etc. which device has solder bumps, solder paste is fed on the substrate and that bonding portions are formed by the fed solder paste and the solder bumps. In prior arts, the solder paste and the solder bumps are formed of the same material, and they are usually melted completely so that the soldering may be achieved. However, in the invention, the solder bumps are not deemed to be a material for soldering but are deemed to be only pads, and the soldering of the pads (the solder bumps) is performed by the solder paste. Further, in the invention, the solder bumps are made of a material having a melting point higher than that of the solder paste so that each of the solder bumps may not be completely melted positively. As stated above, the temperatures of the bumps and the surrounding thereof are apt to become lower than the set reflow temperature, however, the solder paste is readily melted in comparison with the bumps, so that it is easy to perform the soldering.
Thus, even at a reflow temperature at which each of the solder bumps is not melted completely, it becomes possible to realize, while taking the heat resistance of the circuit substrate and electric parts into consideration, soldering of high reliability in a case where bonding portions can be formed by the melting of the solder paste. For example, in a case where a semiconductor device having bumps as the outer pads thereof is mounted on a substrate by use of solder paste for mounting, the solder bumps are made of a material such as Snxe2x80x94Cu alloy or Snxe2x80x94Agxe2x80x94Cu alloy which material has a melting point higher than that of conventional solder containing Pb, the solder for mounting the substrate being made to have a composition of a melting point lower than that of the solder bumps, and the reflow thereof is performed at such a temperature as the heat resisting properties of the substrate and the electric parts are taken into consideration, so that a mixture layer is formed between each of the solder bumps and the solder paste for mounting although each of the solder bumps is not melted completely with some of the original shape of each bump remaining, whereby it becomes possible to perform the bonding of both of the semiconductor device and the circuit substrate with high reliability. In this case, in order to prevent unexpected intermediate products from occurring during the forming of the mixture layer, it is preferred that the solder bumps have a solder composition of the same kind as that of the solder for mounting.
Further, in a case of using the solder of the Snxe2x80x94Agxe2x80x94Cu alloy, there occur such problems as acicular crystals occur and as migration and short are caused due to the acicular crystals. Thus, it is desired to reduce the content of Ag contained in soldering portions formed between the semiconductor device and the circuit substrate. However, in the invention, the solder for mounting needs to have such a composition as to be melted at the reflow temperature, so that the composition of the solder is decided by the reflow temperature, that is, there is a limit regarding the reducing of the Ag content of the solder for mounting. Thus, it is preferred that, by reducing the amount of Ag inflowing from the solder bumps into the solder for mounting, the content of Ag contained in the soldering portions is prevented from increasing. Namely, it is preferred that the content of Ag contained in the solder bumps is less than that of the solder for mounting. For example, the solder balls (i.e., solder bumps) may be made of Snxe2x80x94Cu alloy solder. As regards the solder for mounting, the composition thereof is decided in accordance with the reflow temperature, and a Snxe2x80x94about 2.0 to about 3.5 mass % Agxe2x80x94about 0.3 to about 0.8 mass % Cu alloy is preferred in view of the reliability of bonding. In this specific case, it becomes preferred that the solder balls have the composition of Snxe2x80x94zero to about 2.0 mass % Agxe2x80x94about 0.3 to about 0.8 mass % Cu.